Which among the following polymers has high tensile strength and is used to obtain tyre cords?

The synthetic polymer which resembles natural rubber is

Poly-$\beta$-hydroxybutyrate-co-$\beta$-hydroxyvalerate $(PHBV)$ is a copolymer of . . . . . .

Which one of the statements regarding $X$ is not correct?
$3$-Hydroxybutanoic acid + $3$-Hydroxypentanoic acid $\longrightarrow X$

Natural rubber is a polymer of

If $\bar{M}_w$ is the weight average molecular weight and $\bar{M}_n$ is the number average molecular weight of a polymer, the poly dispersity index $(PDI)$ of the polymer is given by